Heretofore, electrolytic electrodes using valve metals such as titanium as a substrate have been used as excellent insoluble metallic electrodes in the field of electrochemistry and in particular, have been widely used as chlorine-producing anodes in the salt-electrolytic industry.
The term "valve metal" is used herein to indicate titanium, tantalum, niobium, zirconium, hafnium, vanadium, molybdenum, and tungsten.
Metallic electrodes of the above type are well known as described in, for example, U.S. Pat. Nos. 3,632,498 and 3,711,385 and are produced by coating metallic titanium with various electrochemically active materials such as platinum group metals and the oxides thereof. They retain a low chlorine overvoltage for long periods of time as electrodes for the production of chlorine.
However, when these metallic electrodes are used as anodes in electrolysis for the production of oxygen, or in electrolysis accompanied by the generation of oxygen, a serious problem arises in that the anodic overvoltage gradually increases and, in extreme cases, as a result of passivation of the anode, it becomes impossible to continue the electrolysis. Passivation of the anode is believed to be caused mainly by the formation of less conductive titanium oxides resulting from the oxidation of the titanium substrate with oxygen liberated from the metal oxide per se coated on the substrate, or by penetration of oxygen or electrolyte through the electrode coating. Furthermore, since these less conductive oxides are formed in the interface between the substrate and the electrode coating, the adhesion of the electrode coating to the substrate is deteriorated resulting in the electrode coating peeling off and finally in breakdown of the electrode.
Electrolytic processes in which oxygen is produced at the anode, or in which oxygen is generated at the anode as a side reaction include electrolysis using a sulfuric acid bath, a nitric acid bath, an alkali bath or the like; electrolytic recovery of chromium, copper, zinc and the like; electroplating; electrolysis of dilute salt solutions, sea water, hydrochloric acid or the like; and electrolysis for the production of chlorate. All are industrially important.
In these applications, however, serious problems as described above occur in the use of metallic electrodes.
In order to overcome these problems, U.S. Pat. No. 3,775,284 discloses a method of providing a barrier layer comprising a platinum-iridium alloy and oxides of cobalt, manganese, palladium, lead, and platinum between an electrically conductive substrate and an electrode coating to thereby prevent the passivation of electrodes due to penetration of oxygen.
The barrier layer prevents diffusion and penetration of oxygen during electrolysis to a certain extent. The substances forming the barrier layer, however, are electrochemically active and react with electrolyte penetrating through the electrode coating, forming electrolytic products such as gas on the surface of the barrier layer. The formation of these electrolytic products gives rise to additional problems in that the adhesion of the electrode coating is deteriorated by the physical and chemical action of products and the electrode coating may peel and drop off. Furthermore, sufficient durability can not be obtained.
In addition, U.S. Pat. No. 3,773,555 discloses an electrode in which a substrate is coated with a layer of oxide of titanium or the like and a layer of a platinum group metal or oxide thereof laminated on each other. This electrode, however, also suffers from the disadvantage that when it is used in oxygen generation electrolysis, passivation will occur.